The present invention relates to methods of manufacturing marking structures. More particularly, the present invention relates to methods of manufacturing photopolymer marking structures.
It is known to manufacture photopolymer printing plates by exposing a photosensitive material to ultraviolet (xe2x80x9cUVxe2x80x9d) light. It is also known to use a negative film having transparent areas corresponding to a desired image pattern, where the UV light will pass through and cure the photopolymer to produce relief image areas of a desired photopolymer printing plate. The purpose of UV light is to cure, or at least partially cure, exposed areas of the photopolymer material. Other methods of curing desired areas of photopolymer printing plates have been used to manufacture marking devices and other devices made of photopolymer materials.
After the relief image areas are created through the use of negative film and UV light exposure, or other means, it is known to develop the exposed image areas by removing the photopolymer from the non-image areas (corresponding to black, nontransparent areas of the negative film). This can be done by washing the newly created photopolymer plate in a water/surfactant medium. Optionally, an air knife can be used as part of a polymer recovery step prior to washing the photopolymer printing plate.
It is common to then subject the partially manufactured photopolymer printing plate to a post exposure process which involves submerging it in a water based solution and then irradiating it with UV light.
A substantial problem with known methods of manufacturing photopolymer printing plates is that the finished product often has too much surface tack. This problem is particularly prevalent for photopolymer printing plates prepared from liquid resins. The surface tack problem is due, in part, to the inhibition by oxygen of the acrylate polymerization process at the plate surface. It is believed that all commercially available photopolymer printing plates rely on an acrylate polymerization mechanism to create the plate relief and characters.
Efforts have been made to overcome the problem of unwanted surface tack on photopolymer plates. However, all such efforts have been largely unsuccessful or suffer from additional problems.
For example, it is known to incorporate hydrogen abstracting photoinitiators, such as benzophenone, into the photopolymer formulation. It is also known to incorporate amine synergists into the photopolymer material. There are at least two problems associated with the incorporation of such compositions into the photopolymer material. First, photoinitiators are powerful absorbers of UV light especially at the surface of the photopolymer printing plate. This often results in undercutting of the plate characters. A second problem is that the addition of a hydrogen abstracting type photoinitiator can render the plate more susceptible to attack and subsequent degradation by stray UV light (e.g., daylight).
Long-chain fatty acids, such as myristic and palmitic acids, have also been added to photopolymer formulations in an attempt to manufacture a product free of surface tack. Such long chained fatty acids tend to bloom to the surface of the printing plate where a waxy layer is formed thereby rendering the surface substantially tack free. Although the incorporation of long-chain fatty acids may be effective to cover up and substantially eliminate surface tack, such approach has a substantial disadvantage in that it reduces the plate wetting tension and thus, results in poor pick up and transfer of ink.
Another way that the prior art has attempted to reduce surface tack is through the use of germicidal lamps at the post exposure stage as well as standard actinic lamps. This approach also has disadvantages. Namely, it is necessary to use an additional machine which adds substantial costs to the process. Further, the germicidal lamps emit short wave UV radiation that is harmful to the bulk physical properties of the photopolymer printing plate.
Other approaches at the post exposure stage include the incorporation of sodium sulfite, a salt, into a water solution to facilitate the removal of oxygen from the water. The intent of this approach is to again reduce the surface tack. However, the water/sodium sulfite solution has proven to be only marginally successful at reducing surface tack of the manufacturing photopolymer printing plate.
Another post exposure approach has been to add a surfactant into the post exposure water solution in an effort to reduce surface tack. It has been observed that this approach is successful when long-chain fatty acids are also added to the photopolymer formulation itself as discussed above. However, the use of surfactants in a water based solution have proven to be only marginally successful in reducing surface tack of the manufactured photopolymer printing plates.
A new method of manufacturing photopolymer marking structures is needed where the manufactured marking structure has a substantially tack-free surface but nonetheless has desirable ink pick up and transfer qualities.
The present invention overcomes the problems associated with prior art methods by providing new methods of manufacturing photopolymer marking structures that have a substantially tack-free surface while retaining desirable ink pick-up and transfer qualities.
In accordance with one aspect of the present invention, a method of manufacturing marking structures is provided. The method comprises forming a marking structure from a photopolymer material and subsequently placing the marking structure in a solution comprising water, a reducing agent and a surfactant.
It is preferable to expose the marking structure to UV light when the marking structure is immersed within the solution.
It is preferable for the reducing agent to comprises a salt such as sodium sulfite. However, various other reducing agents may be used in accordance with the scope of the present method.
The surfactant preferably comprises an alcohol sulfate such as sodium lauryl sulfate. One commercially manufactured alcohol sulfate is marketed under the trademark EMPICOL LXV. The surfactants used in the present invention presumably suspend or otherwise dissolve the uncured polymer into the aqueous phase, thus making it possible to clean the marking structure leaving it substantially free of uncured photopolymer.
In accordance with a preferred step of the present method, the solution in which the photopolymer marking structure is placed may comprise between about 70%-99.9% of water by weight. The solution may also comprise between about 0.01%-15% of the reducing agent by weight. It is also preferable for the solution to comprise between about 0.01%-15% of the surfactant by weight. The marking structure is preferably at least partially cured in the solution by exposure to UV light, other irradiation sources or other curing means such as chemical catalysis. It is a desired quality of the solution to substantially reduce or eliminate surface tack that would otherwise exist on the marking structure.
In a preferred embodiment, the solution may comprise between about 1%-5% of the reducing agent by weight and between about 1%-5% of the surfactant by weight.
In accordance with a preferred method of manufacturing marking structures, a desired amount of a photopolymer material is placed into a marking structure manufacturing device when the photopolymer material is in an uncured liquid state. The marking structure manufacturing device in which the photopolymer material is cured may include identified desired image areas while other areas are shielded. It is also preferable to expose the photopolymer material arranged at the desired identified image areas to an irradiation source whereby the exposed image areas become partially cured. Preferably, the irradiation source is a UV light source. However, in an embodiment where a light source is required, irradiation sources other than an UV light source may be used. It is also preferred to remove uncured portions of the photopolymer material after the photopolymer material has been exposed to the light source so that the partially cured image areas remain. The partially cured photopolymer marking structure are then placed in a solution comprising water, a reducing agent and a surfactant to remove surface tack. UV light or other curing means is preferably applied to the marking structure when it is in the solution to further cure it.
In a preferred and important aspect of the present invention, a solution comprising a reducing agent such as sodium sulfite, a surfactant and water is used as a post exposure solution in which the partially cured photopolymer marking structure is placed after such marking structure is at least partially cured through exposure to UV light or other curing means. Such a solution is significant for its contribution through the manufacturing process of creating a photopolymer marking structure having a substantially tack-free surface.
The present invention will be more fully understood when the following claims are considered in conjunction with the following detailed description of the preferred embodiments and associated drawings.